The invention relates to an X-ray generator comprising a correction device which provides automatic correction of at least one dose-determining exposure parameter in dependence of the deviation of a measurement value from a predetermined value and which acts on an adjusting member for the exposure parameters.
An X-ray generator of this kind is known from German patent document No. 19 44 481. In the known X-ray generator, the dose power is measured during the exposure and the switch-off dose (and also the voltage applied to the X-ray tube) at which the radiation is switched off by an automatic exposure device is changed in dependence of the deviation of the measurement value from a predetermined value. The sets of exposure parameters preset for different human organs can thus be automatically adapted to the absorption or the thickness of the patient.
Tests have demonstrated that, when taken over a prolonged period of time (a few days), the quality of radiographs is not reproducible. For example, when a correct exposure of a patient is repeated with the same exposure parameters, an incorrect exposure could be obtained in given circumstances. This effect is caused by fluctuations in the image recording system which is formed by film, foil and film-processing. Such fluctuations could occur in the chemical composition of the various baths required for film processing, but could also be temperature fluctuations of, for example, the development bath, fluctuations of the level of the baths, or fluctuations of the processing time. When viewed over a shorter period of time, i.e. one working day, these fluctuations are comparatively small, so that their effect is not yet disturbing; however, from one working day to another the parameters may fluctuate to such an extent that incorrect exposures occur. Moreover, there are fluctuations in the film sensitivity, for example, when a fresh film package is opened or when a film of a different manufacturer is used. Generally, such fluctuations are noticed only after a number of incorrect exposures and they cannot be satisfactorily compensated for according to the present state of the art.